The present Invention relates in general, to a fiber optic gyroscope and, more particularly, to a fiber optic gyroscope utilizing an unpolarized light source to provide an unpolarized optical input.
The state of the art in fibre-optic gyroscopes is approaching theoretical limits for rotation sensing, but the requirements for active control of noise sources and sensitivity tends towards systems with complicated components and electronics. In contrast, the concept of a totally passive fibre gyroscope, without such active control, has appeal for applications owing to the resulting component simplicity and small volume requirements. Many applications for such a device only require moderate rotation rate capabilities of 1-10 degrees per hour. The original demonstrations of the fibre gyroscope were passive, but were limited theoretically in that the device sensitivity approached zero for small rotation rates.
U.S. Patent Application Ser. No. 320,999, now U.S. Pat. No. 4,440,498, by Sheem discloses a passive fiber optic gyroscope utilizing a (3.times.3) directional coupler as an optical input/output device. This gyroscope automatically operates at the maximum sensitivity or quadrature point at zero rotation rate. Additionally, the device exhibits low noise levels, in the few degrees per second range.
Subsequent improvements in single- mode fiber coupler fabrication have occurred which have resulted in still lower noise levels. Additionally, it is now recognized that a significant portion of the observed fiber optic gyroscope noise is due to fluctuations in the Rayleigh backscattered light. This source of noise can be greatly reduced by changing from a single frequency optical input to a broadband input.
Another source of noise in fiber optic gyroscopes is due to "polarization noise" which results from the use of polarized optical sources to drive these devices. The present Inventors have determined that a significant reduction in the polarization noise can be achieved in a (3.times.3) optical gyroscope with no reduction in performance or increase in other forms of noise by the use of an essentially unpolarized broadband optical source to drive the device.